Does the sparkplug fire on the exhaust stroke, in a four stroke? If it does an engine can be taken apart and reassembled 180* out. If it doesn't fire every TDC how does the magneto tell the coil when to fire?

 

Lots of motors are the waste spark variety. But the ones which fire at the proper tdc have a trigger on the cam, which is 1/2 speed of the crank, and fires every revolution of the cam.

 

There is no trigger I can see on the cams. I am dealing with a Z 400, that got a new top end. It idles great but starts cutting at about half throttle. I wondered if it was possible to be 180 off? I guess it must be fine, as far as initial timing.

 

I dont even think its half throttle?!! Could it come how be connected to the reverse revlimmiter?It sounds exactly the same.

 

Id start with jetting, because theres no feedback loop to the cdi , about throttle position. The computer doesnt know about throttle position..so if it were timing or some spark related issue...it would do it all the time and not at specific throttle positions.
If its mainly happening at 1/2 throttle, check the needle position , and or main jet size.

 

We changed main jets, drastically up and down and there was no change. I guess it could be the needle. I am willing to try any suggestions, because we are out of ideas.